Signaling system



May 30, 1939. M E. KROM 2,160,507

SIGNALl-NG SYSTEM Filed Nov. 13, 1956 Patented May 30, 1939 Price SIGNALING SYSTEM Myron E. Krom, Madison, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 13, 1936, Serial No. 110,609

7 Claims.

This invention relates to improvements in signaling systems and more particularly to means for preventing dial impulses, or battery reversal surges, from affecting alternating current responsivefsignal devices associated with telephone lines.

An object of the invention is to effect economies in, and improve the eiciency of telephone trunk circuits.

1G Another and more specific object of the invengo A feature of the invention whereby the foregoing objects are attained resides in a high resistance negative temperature coeicient bridge across the line between the mid-point of which and ground the alternating current responsive g device or ringing relay is connected. By this arrangement direct current flowing in opposite directions in the two sides of line does not aiTect the ringing relay but when alternating current is projected over either side of theline, or both sides 30 simultaneously, from a grounded source, it will,

after a fraction of a second delay, find a low resistance path to ground through the ringing relay or other responsive device.

More specifically the bridge consists of two serially connected resistance units of boron, silver sulphide or other negative temperature coefcient substance, each resistance having a cold or normal resistance of the order of 100,000 ohms and so arranged that the application of ringing 40 current thereto will cause the resistance of each unit to drop to approximately 1,000 ohms in approximately one-half second. This characteristic, i. e., the time interval before the resistance drops to a current conducting value prevents false 45 operation of the ringing relay-on line surges which persist for only Va few milliseconds. Further, the normally high resistance of the bridge does not introduce any inductive disturbance in the' circuit which will distort rapid .59 direct current dial pulses owing in opposite directions in the two sides of the line.

In practice the bridge also includes a pair of condensers one in series with each resistance thereby completely eliminating any steady direct .55 current leakage across the line.

Another feature of the invention resides in the employment of a single wound ringing relay arranged when operated to lock itself vin the operated condition over its operating winding, under control of the answering jack, without affecting :5 the other normal functions of the line.

'I'he invention will be understood from the following description when read in connection lwith the accompanying drawing which represents a localV subscribers station and line, a trunk circuit -10 from a machine switching central oce which trunk is arranged in accordance with the present invention, and a private branch exchange cord circuit for interconnecting said subscribers line and trunk.

Incoming call Let it be assumed that the line terminals of trunk circuit l have been seized at the central oice CO by brushes of an automatic switch and ringing current has been applied over relay contacts 3 and 4 to signal the operator at the private branch exchange. Under this condition a source ofgrounded alternating current, of ringing frequency, is transmitted over the ring conductor vR 2r of the trunk thereby causing operation of the ring-up relay 2 in a circuit which can be traced from the grounded source of ringing current, contacts 4, the ring conductor R of the line, condenser M, negative temperature coefficient resistance element 5' and winding of relay 2 to ground through the locking battery B.

Resistance unit 5' is composed of somematerial such as boron, silver sulphide or their equivalent and of such dimensions as to have a .35 very high resistance, i. e., of the order of 100,000 ohms, under normal conditions, but having the characteristic of decreasing its resistance very rapidly to a comparatively low value when subjected to ringing current of the voltage commonly .40 used in telephone systems, i. e., of the orderof 90 volts.

In the present case an element of boron, whose dimensions are such as to vhave a normal (cold) resistance of 100,000 ohms will, when subjected to 90 volt ringing current for a period of one-half second, lower its resistance to about 100 ohms. Therefore, when ringingcurrent has been applied to the resistance element 5 for approximately one-half second, its resistance will be sufficiently low to permit the required current to ilow through the winding of relay 2 to cause its operation.

Due to the fact that direct ground is connected to the tip conductor T of the trunk over contacts 3, no voltage will be impressed on resistance element 5, which is substantially identical to element If, however, under certain conditions the connection of the ringing source to the line should be reversed, the resistance element 5 would decrease its resistance and allow sucient current to operate relay 2 in the same manner as previously described.

Further, should conditions be such that the ringing source is connected to both the tip and the ring of the trunk in parallel, the resistance of both elements 5 and 5 would decrease to a low value within the specified interval of time.

Relay 2 in operating closes its outer left contacts thereby lighting line lamp II in an obvious circuit.

Closure of the inner left contacts of .relay Y2 completes an obvious locking circuit for itself, which includes battery B.

Condensers I4 and 6 prevent any direct current leakage through the bridge.

When "the private ybranch exchange operator notices the -lighted ylamp I-I, :she inserts plug I2 of her cord circuit into the trunk jack which opens the locking circuit .for 4relay 2 which releases thereby extinguishing lthe line lamp. Insertion ofthe plug "alsocloses contacts I3 thereby completing the ring talk-ing conductor vof `thecord through vto ythe trunk .and `removes vcondenser A from its normally :bridged connection across .the tip `.and ring of 'the trunk. 'The function :of :this condenser vis to normally provide an instantaneous alternating current path of low impedance in bridge -o'f the trunk which is vrequired )for certain :test `purposes in connection with -so-called cross-bar switching systems, which path not provided by condensers 11M and 6 and resistance elements 5 and 5', due to the normally high resistance of elements 5 and 5.

Re-rz'ng Yby central .office I-f for `any reason the central -oice vre-.rings on the trunk before :the plug -I-2 'is removed from the jack, relay 2 again operates approximately one half :second -af-ter ringing starts, which Ytime is required for the unit 5 to reduce its resistance -suiciently for the relay to operate.

When relay 2 operates, it remains operated, and lights lamp I.I, only as :long .as the ringin current is applied to the trunk.

Outgoing call In case subscribers lrequests a :connection With a central :ofi-ice subscriber, .the foperator will insert plug I2 in the trunk jack andas the ring spring is disconnected from the `ring 'conductor of the trunk, at contacts I3, until .the tip of :the -plug fully engages the tip vspring lof -the jack, the .possibility of .a false impulse `of current -over gthe trunk -to the-central .olice is almost negligible.

Insertion of `the plug opensthe locking pathof relay 2 and connects battery through resistance lamp I1 over contacts I of the jack to the 'sleeve ofthe plug to operate the sleeve relayof .the cord (not shown) If the .operator is to dial the wanted number, she-operates the talk and-dial key I'and actuates the -dial I9 lto transmit .the requisite code :of impulses.

Due to the normal high resistance and noninductive `characteristic of the resistance units 5 and 5", the ringing bridge will have no distorting effect on the current impulses in the trunk caused by the rapid interruption (20 per second) of the circuit at the contacts of dial IQ.

What is claimed is:

1. In a signaling system a line, a grounded source of alternating ringing current adapted to be connected to said line, a negative temperature coefficient resistance in bridge of said line, and an alternating current responsive device connected between the mid-point of said bridge and ground.

2. In a signaling system a line, a grounded source of alternating ringing current adapted to be connected to said line, a bridge comprising a pair of boron resistances connected in series across said line, and a ringing current responsive device connected between the junction of said units and ground.

3. In a signaling system a line, a grounded source of alternating current adapted to be `connected to said line, a device responsive to said ringing current, a pair of boron resistors, a vpair of condensers and a connection `between each side of the line and ground including one of said resistors, one of said condensers and said `responsive device.

4. In a telephone system, a trunk line terminating at -one end in an automatic exchange and yin ga Idial .at the other end, a grounded source of alternating ringing current adapted to be conynectedt-o the line at the automatic exchange, a bridge across the -line intermediate the Yends thereof, 4serially comprising a pair of boron resistors and a pair of condensers and a ringing relay connected between ground and the -elecytrical Amid-point of said bridge.

5. In a signaling system a line, a grounded source of ralternating ringing `current adapted to be connected to said line, a negative temperature c'oecient resistance in bridge `of said line and `a single vwound 'alternating 'current relay Aconnected between the mid-point of said bridge and a grounded battery, said relay being so 'arranged that when operated it. closes a locking circuit for itself including its operating winding Y and said groundedbattery.

`6. In a telephone system, -a Vtrunk line and a single wound ringing relay connected thereto, said relay being so arranged and connected Athat when operated, it closes a llocking circuit for itself including its operating winding without afrecting any of the normal functions of the'trunk circuit.

"7. In a telephone system, a trunk `line terminating at-one end in al1-automatic exchangeand in a-dial at the other end, a grounded source 'of alternating ringing `current adapted'to be connected to the line at the automatic exchange, a bridge across the line intermediate the ends thereof serially comprising a pair of 'boron resistors and a pair of `condensers and a single winding relay connected between grounded battery and the electrical Amid-point lof rsaid bridge, said relay being so arranged that when operated it Acloses a locking circuit for itself including its operating winding.

' MYRON E.'KROM. 

